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9: SHEARDESIGN OF WALLS SUBJECT TO IN-PLANE SHEAR
Background
In most cases, shear walls are subject to a combination of shear
and vertical forces. Thecompressive force in load bearing walls
increases shear resistance. In external non-loadbearing
walls, tensile stresses may be present in all or part of the
walls. This will have an adverse effect
on shear resistance and may lead to tensile cracking at wall
heels.
Shear cracks may occur as diagonal steps through bed and header
joints in the centre of a
wall. The slope of the crac k depends on the magnitude and the
type of the applied axialforces.
Shear capacity is usually critical for walls subject to seismic
loading. Spec ial attention should be
given to walls with large openings, to parapets walls and free
standing walls. Refer to AS1170.4
for design requirements of structural and non structural
masonry.
Legend and symbols used in shear design
Adw = the combined bedded area and the grout area where
applicableΦ = the capacity reduction fac tor for shear (0.6
for unreinforced, 0.75 when reinforced)
ƒ ’ms = the characteristic shear strength (refer to
Section 3, Table 3.3 of this manual)
ƒ d = minimum design compressive stress on the
bed joint under consideration (< 2MPa)kv = Shear factor
(refer to Sec tion 3, Table 3.4 of this manual)
Vd = design shear force
Design for Shear using AS3700
AS3700: C lause 7.5.1 expresses wall shear capacity as the
addition of two distinct components;
shear bond strength and shear friction strength, as follows:
Vd ≤ Φ ƒ ’ms Adw +
kv ƒ d Adw
At bedding planes containing damp-proof membranes and at
junctions with other material,
the shear bond term decreases. Shear resistance relies on
applied compressive stresses. This isof concern for non-load
bearing walls under seismic loading, especially where built on
damp-
proof membranes, as they theoretically have no shear
resistance.
The friction term relies on interface material types.
Friction fac tor kv = 0.3 for bed mortar and
decreases to 0.15 for polyethylene and bitumen coated aluminium
type membrane. Refer toSection 3: Table 3.4.
Openings and control joints often require shear connectors and
should be carefully accounted
for in shear design of walls
Design of Shear ConnectorsShear connectors may be used to
transfer shear across mortar joints or to other structural
elements. Refer to AS3700: Clause 7.5.3 for connector design
requirements.
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Design Example:A shear wall, 5m long and 3m
high, constructed on a concrete slab using Alphalite 15-01
hollow
masonry units is subject to a 20 kN/m dead load and has a
racking force of 40kN applied at itstop. Check the capacity of the
wall.
Procedure using AS3700:2001 ƒ ’ms = 1.25 x ƒ
’mt
= 1.25 x 0.2
= 0.25MPa
Bedded area (Adw) = 2 x 25 x 1000
= 50000mm²/m length of wall
kv = 0.3 for bed mortar and for interface with concrete
ƒ d = 20 x 1000/50000= 0.4MPa
Vd =Φ ƒ ’ms Adw +
kv ƒ d Adw= 0.6 x 0.25 x 50000/1000 + 0.3 x 0.4 x
50000/1000
= 7.5 + 6.0= 13.5kN/m
Wall shear resistance = 5m x 13.5kN/m
= 67.5kN
67.5kN > 40kN racking force, so shear design is OK.
Procedure using chartsFrom c hart 9.1, for vertical compressive
force = 20kN/m on a wall 5m long, the load capacity
equals 67kN, which is more than the rac king load of 40kN, so
the design is OK.
Shear Capacity Charts
Charts are based on the following assumptions: Masonry
unit dimensions as discussed in Section 2 of this manual.
Material properties as discussed in Sec tion 3 of this
manual.
Vertical stresses on walls (if applied) are uniform along
walls.
Characteristic shear bond strength = 0.3MPa, assuming
wall does not sit on water proofmembrane, and the shear fac tor kv
= 0.3 Refer to Table 3.4 of this manual.
Further checks should be carried out for local crushing
and or tensile stresses on wall.
Refer to AS1170.4 for unreinforced masonry requirements
for walls subjec t to seismic loads.
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Chart List
Chart 9-1 Shear Capacity of 10-01, 12-01 or 15-01 Masonry Page
4
Chart 9-2 Shear Capacity of 10-31 Solid Masonry Page 4
Chart 9-3 Shear Capacity of Twinbrick Masonry Page 5
Chart 9-4 Shear Capacity of 15-401 or 20-401 Masonry Page 5
Chart 9-5 Shear Capacity of 15-42 Core filled Masonry Page 6
Chart 9-6 Shear Capacity of 15-48 Core filled Masonry Page
6
Chart 9-7 Shear Capacity of 20-01 Hollow Masonry Page 7
Chart 9-8 Shear Capacity of 20-42 or 20-48Core
filled Masonry Page 7
Chart 9-9 Shear Capacity of 30-48 Hollow Masonry Page 8
Chart 9-10 Shear Capacity of 30-48 Core filled Masonry Page
8
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